The context of NGOA (Next Generation Optical Access) and PON (Passive Optical Networks) is known fin the art. The ONUs are located in the subscribers' homes, while the OLT is located in the central office.
FIG. 1 shows an ONU with the connection to the PON at the left. The numbers are taken from the use case, where attenuation in the PON is at maximum and the receive level at the ONU is just −45 dBm. The Tx signal has to have maximum strength in this case to overcome the PON attenuation in upstream direction. Therefore this is the worst case for near-end crosstalk: The Rx signal is weak and the Tx signal is strong.
In FIG. 1 the near-end crosstalk is assumed to be −20 dB. Such a back reflection in a connector has no significant influence to the transmission of the signals; especially the attenuation due to a bad connector can be neglected. Please note, that other systems like GPON are not vulnerable to near-end crosstalk and connector reflections. If, for example, existing GPON PONs are migrated to NGOA the operator may experience problems, because some of the new NGOA ONUs may not work after migration.
Reflections at connectors are specified by the suppliers of the optical connectors. Nevertheless these specifications are only valid, if the connectors are cleaned before they are plugged. In the field the operator cannot guarantee, that these cleaning procedures are done perfectly. This problem is increased in fiber-to-the-home applications, where the end user does the last connection manually.
This may have the following consequences:                The ONU may not be able to establish a bi-directional connection to the OLT due to near-end crosstalk.        The ONU may not be able to detect the source of the problem.        The ONU may not be able to inform the OLT about the problem; the operator has no hint 42 where the problem is located in the field.        The tolerance to near-end crosstalk of an NGOA ONU may be too low for real-world applications.        
In general there are three principles to avoid near-end crosstalk (NEC):                Wide spectral distance between Rx and Tx spectrum. For example Rx and Tx can be in different optical bands. This allows filtering in the optical domain to suppress NEC. GPON is an example.        Two fibers. Rx and Tx happens at different fibers. Long-haul systems are an example.        Ping-Pong: Rx and Tx happens at different time intervals.        